Experimental solidification of an andesitic melt by cooling

Abstract

Solidification experiments at (a) five different cooling rates (25, 12.5, 3, 0.5 and 0.125 °C/min) between 1300 and 800 °C, and (b) variable quenching temperatures (1100, 1000, 900 and 800 °C) at a fixed cooling rate of 0.5 °C/min were performed on an andesitic melt (SiO2=58.52 wt.% and Na2O+K2O=4.43 wt.%) at air conditions from high superheating temperature. The results show that simultaneous and duplicated experiments with Pt-wire or Pt-capsule produce identical run-products. Preferential nucleation on Ptcontainers or bubbles is lacking. Plagioclase and Fe–Ti oxide crystals nucleate firstly from the melt. Clinopyroxene crystals form only at lower cooling rates (0.5 and 0.125 °C/min) and quenching temperatures (900 and 800 °C). At higher cooling rates (25, 12.5 and 3 °C/min) and quenching temperature (1100 °C), plagioclase and Fe–Ti oxide crystals are embedded in a glassy matrix; by contrast, at lower cooling rates (0.5 and 0.125 °C/min) and below 1100 °C they form an intergrowth texture. The crystallization of plagioclase and Fe–Ti oxide starts homogeneously and then proceeds by heterogeneous nucleation. The crystal size distribution (CSD) analysis of plagioclase shows that crystal coarsening increases with decreasing cooling rate and quenching temperature. At the same time, the average growth rate of plagioclases decreases from 2.1×10−6 cm/s (25 °C/min) to 5.7×10−8 cm/s (0.125 °C/min) and crystals tend to be more equant in habit. Plagioclases and Fe–Ti oxides depart from their equilibrium compositions with increasing cooling rate; plagioclases shift from labradorite–andesine to anorthite–bytownite. Therefore, kinetic effects due to cooling significantly change the plagioclase composition with remarkable petrological implications for the solidification of andesitic lavas and dikes. The glass-forming ability (GFA) of the andesitic melt has been also quantified in a critical cooling rate (Rc) of ~37 °C/min. This value is higher than those measured for latitic (Rc ~1 °C/min) and trachytic (Rcb0.125 °C/min) liquids demonstrating that little changes of melt composition are able to significantly shift the initial nucleation behavior of magmas and the following solidification paths.